The invention relates to a process for preparing a, solid organic-inorganic hybrid polymer electrolyte containing lithium ions. The product shows high strength, conductivity and lithium transference values. Further, the product can be self-organized into nanometer scale plates and rods paving the way to making lithium conducting cables for example and hence batteries of nanometer size.
The good conductivity in polymer electrolytes occurs essentially because their glass transition temperatures Tg are below ambient. Therefore, the mobility of both the ions and the polymer segments is high, and indeed polymer segmental motion is critical to good conductivity in these materials (Shriver and Bruce, Polymer electrolytes I: general principles. In: Solid State Electrochemistry, Bruce, P. G., ed., Cambridge 1997). PEO is excellent as a polymer host because, its Tg is low (xe2x88x9240xc2x0 C.), and it coordinates cations well enough to dissolve lithium salts (lightfoot et al., Science 262 (1993), 883). To be practical as an electrolyte, however, the resulting material must be stabilized against crystallization without sacrificing conductivity and without degrading its mechanical properties.
Composites have been developed recently that address these issues in different ways (Meyer, Adv. Mater. 10 (1998), 439; Croce et al., Nature 394 (1998), 456-458; Gianellis, Adv. Mater. 8 (1996), 29-35; Quartarone et al., Solid St. Ionics 110 (1998), 1-14). One approach is to cross-link the polymer, which suppresses crystallization and improves mechanical performance. The disadvantage of this approach is that Tg is raised, so that the conductivity at ambient temperature is reduced. The cross-linked material can be additionally plasticized, but usually the resulting gels are of modest electrochemical stability and many of the mechanical advantages are lost. A different, synthetically challenging, approach is to graft ethylene oxide oligomers onto stiff polymer backbones (Meyer (1998), supra). A layered architecture with excellent mechanical properties and supressed PEO crystallization results, but with marginal electrochemical stability.
A layered structure is also achieved in polymer-inorganic intercalate materials, which have the additional advantage of fixed counterions, and therefore a high transference number for the cations. Such intercalates have to been made with both PEO (Gianellis (1996), supra; Lemmon et al., Electrochim. Acta 40 (1995). 2245-2249) and poly-phosphazenes (Hutchison at al., Chem. Mater. 8 (1996), 1597-1599), a problem with this approach is the anisotropy of the resulting conductivity. The recently synthesized siloxyaluminate polymers represent a potentially major is improvement in this area, since they appear to yield isotropic materials (Rawsky et al., Chem. Mater. 6 (19941, 2208-2209; Fujlnami et al., Chem. Mater. 9 (1997), 2236-2239). Even when the counterions are not fixed, addition of Lewis acid sites greatly improves the cation transference number (Croce et al., (1998), supra).
Lee et al. (Mol. Cryst. Liq. Cryst. 294 (19971, 229-232) describe processes for the sol-gel preparation of organic-inorganic hybrid polymer electrolytes and their electrochemical characterizations. The polymer electrolytes are prepared based on low molecular weight poly(ethylene glycol), lithium salts and an inorganic matrix produced by a sol-gel process of tetraethoxysilane and a precursor which was synthesized by the reaction of monomethoxy terminated poly(ethylene glycol) and 3-isocyanatopropyltriethoxysilane. Disadvantages of this product are, besides the demanding chemistry, that the silica network is based on tetraethoxysilane that does not provide any sort of Lewis acid site. Therefore the transference number is expected to be low. Furthermore, the modulus of the materials presented will be low because no network hardener like an aluminum compound is present. It will also be difficult to achieve a homogeneous distribution of silica within the sample.
It was therefore an object of the invention to provide a process for preparing a solid hybrid polymer electrolyte which avoids at least some of the disadvantages of the prior art.
This problem is solved by means of a process for preparing a solid hybrid polymer electrolyte comprising the steps:
a) forming a mixture comprising at least one silicon-containing precursor and at least one aluminum-containing precursor wherein said precursors can be reacted to give an organic-inorganic hybrid composite,
b) reacting the mixture from a) whereby a sol is formed by reaction of the precursors.
c) adding a polyalkylene oxide-containing polymer and a lithium salt to the mixture from step a) and/or to the sol from step b) and
d) reacting the mixture from c) whereby a solid organic-inorganic hybrid polymer electrolyte is obtained.
According to the invention a mixture is first formed. which comprises at least one silicon-containing precursor and at least one aluminum-containing precursor. The precursors used may be any desired substances which can be reacted to give an organic-inorganic hybrid composite.
The silicon-containing precursor is preferably a functionalized orthosilicate comprising at least one Sixe2x80x94C bond. More preferably, the silicon-containing precursor is a compound of the formula (I)
xe2x80x83(Rxe2x80x2)nSi(OR)mxe2x80x83xe2x80x83(I)
wherein
each Rxe2x80x2 is independently a straight-chain or branched C1-C10 alkyl group containing a compatibilizing functionality, wherein said compatibilizing functionality is capable of interacting with a polyalkylene oxide-containing polymer,
each R is independently a straight-chain or branched, substituted or unsubstituted C1-C8, preferably C1-C4 alkyl group, e.g. methyl or ethyl,
n is 1 or 2 and
m is 3 or 2, with the proviso that n+m=4.
Preferably, the compatibilizing functionality is capable of forming cligoalkyleneoxy moieties, particularly oligoethyleneoxy moieties bound to silicon atoms. Particularly preferably, the compatibilizing functionality is an epoxide group, e.g 
a group.
Thus, a particularly preferred example of Rxe2x80x2 is a 3-glycidyfoxypropyl group. A particulary preferred example for the silicon-containing precursor is (3-glycidyloxypropyl)-trimethoxysilans.
The aluminum-containing precursor is preferably a compound of the formula (II):
Al(OR)3xe2x80x83xe2x80x83(II)
wherein
each R is indepenendly a straight-chain or branched, substituted or unsubstitued C1-C8, preferably C1-C6 alkyl group, e.g. a methyl, ethyl, propyl or butyl group.
A particularly. preferred example for an aluminum-containing precursor is Al(sec-butoxide).
Step b) of the process according to the invention comprises reacting the precursor mixture whereby a sol is formed. In this step water or an aqueous solution is used to hydrolyze the alkoxy group-containing precursors forming a sol comprising Sixe2x80x94OH and Alxe2x80x94OH groups. Preferably, the hydrolysis of the precursors is carried out under acid-conditions, e.g. In the presence of 0.01 N HCl.
Step c) of the process according to the invention comprises adding a polyalkylene oxide-containing polymer and a lithium salt to the mixture from step a) and/or the sol from step b). The addition of the polyalkylene oxide containing polymer and the lithium salt may occur before and/or after the reaction of the precursors-to the sol. Preferably, the polymer and the lithium salt are added -after the formation of the sol. The polymer and the sol may be added in a solvent in which both components are soluble, preferably an organic solvent, e.g. a chlorinated hydrocarbon or a linear or cyclic ether, particularly preferably chlorform, tetrahydrofuran or mixtures thereof.
The polyalkylene oxide-containing polymer is preferably a polyethylene oxide containing polymer having a molecular weight in the range of from about 100 g/mol to about 10.000 g/mol, more preferably from about 300 g/mol to about 2.000 g/mol. In one preferred embodiment of the invention the polymer is a polyethylene homopolymer. In another preferred embodiment of the invention the polymer is a block copolymer comprising polyethylene oxide blocks and hydrophobic blocks wherein these hydrophobic blocks may be selected from the group consisting of polyisoprene, polybutadiene, polymethylsiloxane, methylphenylsiloxane, polyacrylates of C3-C4 alcolhols, polymethracrylates of C3-C4 alcohols, hydrogenated polyisoprene, polybutadiene and mixtures thereof. Preferably these hydrophobic blocks have a glass transition temperaturexe2x89xa6room temperature, i.e. 25xc2x0 C., particularly preferably xe2x89xa60xc2x0 C. and most preferably xe2x89xa6xe2x88x9225xc2x0 C. Examples of these types of polymers are given, e.g. in WO99/12994, which is incorporated herein by reference.
The lithium salt may be an inorganic or organic lithium salt, wherein organic lithium salts are preferred. More preferably, the lithium salt is selected from lithium carboxylates and lithium sulfonates. The organic lithium salts may comprise halogenatic organic anions, particularly fluorinated organic anions such as the trifluoromethylsulfonate (triflate) anion.
Step d) of the process according to the invention comprises reacting the mixture from c) whereby a solid organic-inorganic hybrid polymer electrolyte is obtained. This step comprises the forming of Sixe2x80x94Oxe2x80x94Si bonds, whereby a three-dimensional network of Si atoms is formed. Further, the reaction preferably comprises the removal of any volatile constituents from the reaction mixture, e.g. by heating and/or vacuum treatment. For details it is referred to WO99/12994.
Further, the present invention refers to a solid hybrid polymer electrolyte which is obtainable by the process as described above. The solid hybrid polymer electrolyte of the invention has a three-dimensional network structure comprising Si atoms bound to 1, 2 or 3 bridging oxygens (i.e. oxygens forming Sixe2x80x94Oxe2x80x94Si bridges), wherein the three-dimensional network structure further contains aluminate ions, lithium cations and a polyalkylene oxide-containing polymer. The molar ratio of Si and Al atoms is preferably in the range from about 99:1 to about 1:1, more preferably from about 2:1 to about 1:1. The molar ratio of Li ions to aluminate anions is preferably in the range of from 0.4 to about 3, more preferably from about 1.3 to about 2. The weight ratio of polyalkylene oxide-containing polymers to the precursor compounds is preferably in the range of from about 0.21:1 to about 1.2:1 more preferably from about 0.7:1 to about 1:1.
Further, it is preferred that at least about 30% to 40% of the Si atoms in the network structure are bound to 3 bridging oxygens. The Al atoms are present as aluminate anions, which may comprisexe2x80x944 and/orxe2x80x946-fold coordinate Al atoms. Preferably, about 35-65%, more preferably about 50% of the Al are 4-fold coordinate atoms.
The polyalkylene-containing polymer in the polyelectrolyte is non-crystalline at room temperature. Preferably, the polymer has a glass transition temperature of less than about xe2x88x9220xc2x0 C., more preferably of less than about xe2x88x9230xc2x0 C.
Further, it i preferred that the polymer electrolyte has a mechanical modulus (as determined by dynamic spectroscopy) of at least about 5xc3x97106 Pa, more preferably of at least about 107 Pa at room temperature. The polymer electrolyte has preferably a conductivity as measured according to dielectric spectroscopy and impedance spectroscopy of at least about 10xe2x88x925 S/cm at room temperature, more preferably up to about 10xe2x88x924 S/cm or more.
The polymer electrolyte may be in the form of an anisotropic solid, e.g. a monolith or a film. The polymer electrolyte, however, may be in the form of a nanostructure such as a plate and/or a rod or cable. The forming of these nanostructures is described in detail in WO99/12994.